7 Sep
2006
7 Sep
'06
4:18 a.m.
Subject: Re: PDP-8m Console Switch Problems - fixed!
From: Don North <ak6dn at mindspring.com>
Date: Thu, 07 Sep 2006 00:49:17 -0700
To: General Discussion: On-Topic and Off-Topic Posts <cctalk at
classiccmp.org>
Cc: cctech at classiccmp.org
None of the databooks I have ever seen actually show the gate level logic
for the internals of the 74x175 or 74x174; they are only black box diagrams.
So it is not clear to me that the outputs are unbuffered/bidirectional.
They certainly seem to be for Signetics (see below) but my lab tests cannot
verify this for any other mfg.
I have old databooks sets that go to the gate and some cases
transistor level.
I have pulled totem-pole outputs forcibly high or low
before, but
have always used an external series current-limiting resistor to limit
the override current to a 'safe' value.
For pullup a current limit is advised, for pull down it's not needed
as the pull up device is current limited already.
Well, I am seeing something very different here. I
built a breadboard
with the circuit under question: 74x175 device, 470ohm to +5V on MR~,
CLK and Dx inputs forced to ground. I connected voltmeters to the
Q and Q~ outputs, and then tried forcing Q and Q~ alternately to ground
to see if I could change the state of the device.
I had a number of devices to test, here are the results:
DATE
MFG DEVICE CODE RESULT
--- ------ ---- ---------
TI 74175 87 FAIL, shorting Q~ to gnd never changes Q to high
SGS 74LS175 82 ditto
TI 74AS175 87 ditto
SIG 74S175 84 PASS, with 470ohm pullup to +5V on Q req'd for Q0-Q3
SIG 74S175 76 PASS, Q1-Q3 work w/ no resistor, Q0 requires 470ohm
For the failed devices, I tried with no pullup, and 100, 470, 1K, 4.7K
pullups to +5V on Qx. No value of pullup made any difference. Shorting
a Q~ signal (at ~4V) to ground never changed Q (at ~0.4V) to a HIGH.
It the MR/ is not in the correct state (may need pullup) I'd conclude
you have some bad parts. Especially the TI[I have the most data on those]!
I'd give
the 7404 the hairy eyeball! A quick test is socket a '175
with the Q and /Q output pins floating and using a jumper to ground
make it flip [It WILL NOT IF MR/ is asserted, you can bend out the
MR/ pin to avoid that.]. Then test the '04 for input changes output.
Allison
The 7404 on the output seems OK, as is the rest of the downstream logic
(the priority encoder). I measured the input currents required on a
suspect '04 input to set the output high and low and they are well within
spec (about +20uA for input high, -0.7mA for input low). With the 74S175
out of its socket I could set all the 7404 inputs H/L and observe the
downstream priority encoder outputs were just as expected. I have about 15 of the 1984 Signetics 74S175s, I tried
all of them in
the console board socket; none of them worked, even a little bit, with
no pullups added.
I'd pulse them slow with Q/ connected to D and see if they toggle.
I suspect you have a bad run of old chips. I just tossed a few tubes
of mid 80s NOS parts as they apprently died of silicon rust [moisture gets
into the plastc and they die].
So I added 470ohm pullups to +5V on the 74S175 Q
outputs to 7404 inputs.
Everything started working as would be expected. The switch decode logic
is now 100% functional.
You have something really messed up with those 175s your testing.
My thought is that the output pullups on the 74S175 Q
pins are trying to
pull those outputs high fairly strongly (10mA load) but the Q output can
still drive a valid low (it is a 20mA schottky driver). This pullup
'preloads' the output, so that a kick on the QB~ (by shorting to gnd)
gets the Q output moving high, the resistor keeps it moving high, able to
override some smaller internal driver trying to keep the output low.
At least that is the only rational explanation I can think of right now.
Sounds like you have some '175s with the upper device fried in the totem
pole outputs.
Just the 7404 by itself is a -1mA low, +40uA high load;
not very much.
Unless it has an input with high leakage to Vcc or ground, i've seen both.
In any event, the fix is simple (three resistors
really, but I'll add one
on each of the six used outputs). Turns out all the original logic chips
appear to be good (I had removed the original 74S175 intact for testing).
Thanks to everyone for all the suggestions and helpful hints.
Put all the parts on a header and plug it in rather than mess the board up.
Allison
7 Sep
7 Sep
7:29 a.m.
New subject: OT: list weirdness (was Re: PDP-8m Console Switch Problems - fixed!)
>Subject: Re: PDP-8m Console Switch Problems -
fixed!
> From: Don North <ak6dn at mindspring.com>
> Date: Thu, 07 Sep 2006 00:49:17 -0700
> To: General Discussion: On-Topic and Off-Topic Posts
<cctalk at
classiccmp.org>
> Cc: cctech at classiccmp.org
I've been following the 8m console topic with interest, but I never seem to
receive Don North's side of the conversation. I've checked email spam
folders
and whatnot, but there's no trace of it anywhere I can find. (Except the
list
archives, of course.)
Anyone know what's going on?
Vince
8:11 a.m.
New subject: OT: list weirdness (was Re: PDP-8m Console Switch Problems - fixed!)
At 7:29 AM -0700 9/7/06, vrs wrote:
I've been following the 8m console topic with
interest, but I never seem to
receive Don North's side of the conversation. I've checked email spam
folders
and whatnot, but there's no trace of it anywhere I can find. (Except the
list
archives, of course.)
Anyone know what's going on?
No, but I noticed a similar problem over the weekend, I didn't have
time to look into it, I thought my ISP's anti-spam filters might be
to blame. Though that seems *highly* unlikely, and I just checked
and don't see any sign of that being the case.
In the thread you're talking about I received to messages that were
from Don North.
Zane
--
| Zane H. Healy | UNIX Systems Administrator |
| healyzh at aracnet.com (primary) | OpenVMS Enthusiast |
| MONK::HEALYZH (DECnet) | Classic Computer Collector |
+----------------------------------+----------------------------+
| Empire of the Petal Throne and Traveller Role Playing, |
| PDP-10 Emulation and Zane's Computer Museum. |
| http://www.aracnet.com/~healyzh/ |
11:28 a.m.
Allison wrote:
That's good.
If I had only
tested a few devices, maybe I would agree. But in total I
tested
12 TI 74175 and 15 SIG 74S175. Each device behaved consistently. I could
never
get a TI '175 to pass (meaning yanking a high Q~ to gnd forced Q high);
and I
could only get the SIG 'S175 to pass with a pullup to +5V on Q. The
'best' S175
I found would work with a 2.2K pullup. To get them all to work reliably
I had
to go down to 470ohm pullup (~10mA current).
I also find it somewhat suspicious in that the BOM for the PDP-8f/m LED
console
panel calls out a plain 74175 device for these parts (rev F board etch).
However
in the three panels which I have, they are all BUILT with SIG 74S175 devices
of various date codes in 75 and 76. None of these parts looks to have been
previously reworked (or else someone did a real nice job; hard to tell
sometimes).
I find it somewhat fishy the BOM and the board have different parts; DEC was
usually a little more reliable than this.
But like I said earlier all the devices pass OK on the IC tester.
To
be clearer, the numbers I quoted just above are not databook numbers but
ACTUAL numbers I measured in circuit on the 7404 in my board (I have access
to all the inputs since I socketed both 74175 parts). I used a small diode
(1N4148) in series with my multimeter to measure input high/low currents.
I plan
to do that, just replacing the two S175 positions with sockets,
and I'll
dead bug the resistors over the top of the S175s. That way, someday when
I come
across some of these 'magic' 74x175s :-) that work without pullups I can
replace
them easily.
Don North wrote:
I had a 470ohm pullup to +5V on MR~ for all the tests, so there was no issue
WRT inadvertent or floating MR~ forcing the Q output low.
Prior to using all the '175 parts I ran them thru my PROM programmer /
IC tester
to validate they are good. All the 74x175s I used tested 'good' at both
4.5V and
5.5V VCC. I even added additional tests (repeated pattern sequences) to the
tune of about 25 sequential vectors total. All the functions (clocking,
clearing, holding data) tested out OK on all the parts I used. I only found
one bad part (an old TI 74175 dc 87; one Q output seemed to be floating).
Well, I am seeing something very different here.
I built a breadboard
with the circuit under question: 74x175 device, 470ohm to +5V on MR~,
CLK and Dx inputs forced to ground. I connected voltmeters to the
Q and Q~ outputs, and then tried forcing Q and Q~ alternately to ground
to see if I could change the state of the device.
I had a number of devices to test, here are the results:
DATE
MFG DEVICE CODE RESULT
--- ------ ---- ---------
TI 74175 87 FAIL, shorting Q~ to gnd never changes Q to high
SGS 74LS175 82 ditto
TI 74AS175 87 ditto
SIG 74S175 84 PASS, with 470ohm pullup to +5V on Q req'd for Q0-Q3
SIG 74S175 76 PASS, Q1-Q3 work w/ no resistor, Q0 requires 470ohm
For the failed devices, I tried with no pullup, and 100, 470, 1K, 4.7K
pullups to +5V on Qx. No value of pullup made any difference. Shorting
a Q~ signal (at ~4V) to ground never changed Q (at ~0.4V) to a HIGH.
It the MR/ is not in the correct state (may need pullup) I'd conclude
you have some bad parts. Especially the TI[I have the most data on those]!
I'd
give the 7404 the hairy eyeball! A quick test is socket a '175
with the Q and /Q output pins floating and using a jumper to ground
make it flip [It WILL NOT IF MR/ is asserted, you can bend out the
MR/ pin to avoid that.]. Then test the '04 for input changes output.
Allison
The 7404 on the output seems OK, as is the rest of the downstream logic
(the priority encoder). I measured the input currents required on a
suspect '04 input to set the output high and low and they are well within
spec (about +20uA for input high, -0.7mA for input low). With the 74S175
out of its socket I could set all the 7404 inputs H/L and observe the
downstream priority encoder outputs were just as expected. I have about 15 of the 1984 Signetics 74S175s, I
tried all of them in
the console board socket; none of them worked, even a little bit, with
no pullups added.
I'd pulse them slow with Q/ connected to D and see if they toggle.
I suspect you have a bad run of old chips. I just tossed a few tubes
of mid 80s NOS parts as they apprently died of silicon rust [moisture gets
into the plastc and they die].
So I added 470ohm pullups to +5V on the 74S175 Q
outputs to 7404 inputs.
Everything started working as would be expected. The switch decode logic
is now 100% functional.
You have something really messed up with those 175s your testing. My thought is that the output pullups on the
74S175 Q pins are trying to
pull those outputs high fairly strongly (10mA load) but the Q output can
still drive a valid low (it is a 20mA schottky driver). This pullup
'preloads' the output, so that a kick on the QB~ (by shorting to gnd)
gets the Q output moving high, the resistor keeps it moving high, able to
override some smaller internal driver trying to keep the output low.
At least that is the only rational explanation I can think of right now.
Sounds like you have some '175s with the upper device fried in the totem
pole outputs.
Just the 7404
by itself is a -1mA low, +40uA high load; not very much.
Unless it has an input with high leakage to Vcc or ground, i've seen both. In any event, the fix is simple (three resistors
really, but I'll add one
on each of the six used outputs). Turns out all the original logic chips
appear to be good (I had removed the original 74S175 intact for testing).
Thanks to everyone for all the suggestions and helpful hints.
Put all the parts on a header and plug it in rather than mess the board up.
Allison
8 Sep
8 Sep
3:23 p.m.
I was curious ... pulled out a bunch of NOS 74x175s
and did some tests. I noticed that the behaviour was
sensitive to Vcc and ended up with the following table.
The values 0<=n<=4 in the matrix indicate the number of
flip-flops in the IC which worked as desired, so 4 is 'good'.
<--> repeat across
--> progression up
<-- progression down
Vcc
Unit MFG DEVICE DATE 4.1 4.3 4.5 4.7 4.9 5.1 5.3
---- --- ------ ---- |---|---|---|---|---|---|---|
1. TI 175 7340 4 4 4 3 3 3 3
2. " 4 <--> 4
3. " 4 4 4 4 3 3 3
4. TI 175 7624 0 <--> 0
5. Hit 175 6G46 4 3 <-- 0
6. TI S175 7340 4 <--> 4
7. " 1 --> 3 4 4
8. " 0 --> 3
9. " 2 3 4 <--> 4
10. TI S175 7936 4 <--> 4
11. " 4 <--> 4
12. " 4 <--> 4
13. " 4 <--> 4
14. NS S175 8742 4 <--> 4
15. NS LS175 8332 0 <--> 0
Observations:
- behaviour may differ between IC units even within valid Vcc range,
- a given IC unit may change its behaviour within or near valid Vcc range,
- 175 class may pass at lower Vcc and fail at higher Vcc,
- S175 class may pass at higher Vcc and fail at lower Vcc.
Interesting the way 175 and S175 devices differ in their response to Vcc change.
The group of TI S175s from 1979 did seem to show reliable behaviour over the
entire range.
One way or the other, the DEC front panel is relying on unspecified behaviour
of the device. I wonder if the designers were just relying on old habits of
setting flip-flops via collector triggerring from the discrete days, and just
got lucky that it worked.
(Another observation was that the clock input *must be held low* for any
of the devices to work as desired.)
5:04 p.m.
Brent Hilpert wrote:
Observations:
- behaviour may differ between IC units even within valid Vcc range,
- a given IC unit may change its behaviour within or near valid Vcc range,
- 175 class may pass at lower Vcc and fail at higher Vcc,
- S175 class may pass at higher Vcc and fail at lower Vcc.
Interesting the way 175 and S175 devices differ in their response to Vcc change.
The group of TI S175s from 1979 did seem to show reliable behaviour over the
entire range.
One way or the other, the DEC front panel is relying on unspecified behaviour
of the device. I wonder if the designers were just relying on old habits of
setting flip-flops via collector triggerring from the discrete days, and just
got lucky that it worked.
(Another observation was that the clock input *must be held low* for any
of the devices to work as desired.)
Very interesting. I hadn't run any tests varying the VCC voltage in my
tests.
I'll have to try doing that on the set of parts I've already tested.
I also went over to ACE today in San Jose and got half a dozen samples
each of
TI, NSC, Fairchild, Signetics 74175 and 74S175 from the late 70s/early
80s to test.
But based on Brent's data alone I may go back and look for TI S175s from
the late 70s.
Don
9 Sep
9 Sep
12:08 a.m.
Brent Hilpert wrote:
I was curious ... pulled out a bunch of NOS 74x175s
and did some tests. I noticed that the behaviour was
sensitive to Vcc and ended up with the following table.
The values 0<=n<=4 in the matrix indicate the number of
flip-flops in the IC which worked as desired, so 4 is 'good'.
<--> repeat across
--> progression up
<-- progression down
Vcc
Unit MFG DEVICE DATE 4.1 4.3 4.5 4.7 4.9 5.1 5.3
---- --- ------ ---- |---|---|---|---|---|---|---|
1. TI 175 7340 4 4 4 3 3 3 3
2. " 4 <--> 4
3. " 4 4 4 4 3 3 3
4. TI 175 7624 0 <--> 0
5. Hit 175 6G46 4 3 <-- 0
6. TI S175 7340 4 <--> 4
7. " 1 --> 3 4 4
8. " 0 --> 3
9. " 2 3 4 <--> 4
10. TI S175 7936 4 <--> 4
11. " 4 <--> 4
12. " 4 <--> 4
13. " 4 <--> 4
14. NS S175 8742 4 <--> 4
15. NS LS175 8332 0 <--> 0
Vcc
Unit MFG DEVICE DATE 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5 Notes
---- --- ------ ---- |---|---|---|---|---|---|---|---| -------------
16. NS 175 7923 4 4 4 4 4 4 3 3
17. NS 175 8948 4 4 4 4 4 2 0 0
18. FAIR 175 8001 4 4 4 4 4 3 1 0
19. SIG 175 8202 4 4 4 4 4 4 4 4 Works in PDP-8m console
20. TI 175 8705 0 0 0 0 0 0 0 0
21. SGS LS175 8241 0 0 0 0 0 0 0 0
22. TI S175 345B 4 4 4 4 4 4 4 4 Works in PDP-8m console
23. SIG S175 8421 0 0 0 0 0 0 0 0 Fails w/ no pullups
Observations:
- behaviour may differ between IC units even within valid Vcc range,
- a given IC unit may change its behaviour within or near valid Vcc range,
- 175 class may pass at lower Vcc and fail at higher Vcc,
- S175 class may pass at higher Vcc and fail at lower Vcc.
Interesting the way 175 and S175 devices differ in their response to
Vcc change.
I didn't see any of the fail at lower VCC on any S175 parts, but I
only
tested one sample from each date code. I have more parts, but I got
bored :-)
The group of TI S175s from 1979 did seem to show reliable behaviour
over the
entire range.
I found this to be true of the TI S175 date code 345B (which I
don't
know how
to decode) and the Signetics 175 from '82. Both were rock solid in the test
circuit and work reliably in the PDP-8m console socket with no add'l rework
(ie, no pullups required as with the previous S175 SIG part).
One way or the other, the DEC front panel is relying on unspecified
behaviour
of the device. I wonder if the designers were just relying on old
habits of
setting flip-flops via collector triggering from the discrete days,
and just
got lucky that it worked.
I agree, this circuit is more complex than it really
needs to be. The
two 175s
and the 04 hex inverter packs can be replaced by three 7400 quad nand
packs (plus
six pullup resistors) that implement simple cross-coupled latch debouncers.
Sometimes a designer gets a bug in their head to implement a 'cute' circuit
when really doing something more simple and straightforward is the
better solution.
Of course I would never do that :-), I'm only reporting that I've seen
it done.
Anyway, for now it appears replacing the existing flaky S175 dc '76
parts in my
PDP-8m console with SIG 175 dc '82 parts has fixed the problems, no
add'l rework
or circuit hacks required.
Don
12:17 a.m.
[ sorry for the resend - the table format was messed
up in the last message - Don ]
Brent Hilpert wrote:
I was curious ... pulled out a bunch of NOS 74x175s
and did some tests. I noticed that the behaviour was
sensitive to Vcc and ended up with the following table.
The values 0<=n<=4 in the matrix indicate the number of
flip-flops in the IC which worked as desired, so 4 is 'good'.
<--> repeat across
--> progression up
<-- progression down
Vcc
Unit MFG DEVICE DATE 4.1 4.3 4.5 4.7 4.9 5.1 5.3
---- --- ------ ---- |---|---|---|---|---|---|---|
1. TI 175 7340 4 4 4 3 3 3 3
2. " 4 <--> 4
3. " 4 4 4 4 3 3 3
4. TI 175 7624 0 <--> 0
5. Hit 175 6G46 4 3 <-- 0
6. TI S175 7340 4 <--> 4
7. " 1 --> 3 4 4
8. " 0 --> 3
9. " 2 3 4 <--> 4
10. TI S175 7936 4 <--> 4
11. " 4 <--> 4
12. " 4 <--> 4
13. " 4 <--> 4
14. NS S175 8742 4 <--> 4
15. NS LS175 8332 0 <--> 0
Vcc
Unit MFG DEVICE DATE 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5 Notes
---- --- ------ ---- |---|---|---|---|---|---|---|---| -------------
16. NS 175 7923 4 4 4 4 4 4 3 3
17. NS 175 8948 4 4 4 4 4 2 0 0
18. FAIR 175 8001 4 4 4 4 4 3 1 0
19. SIG 175 8202 4 4 4 4 4 4 4 4 Works in PDP-8m console
20. TI 175 8705 0 0 0 0 0 0 0 0
21. SGS LS175 8241 0 0 0 0 0 0 0 0
22. TI S175 345B 4 4 4 4 4 4 4 4 Works in PDP-8m console
23. SIG S175 8421 0 0 0 0 0 0 0 0 Fails w/ no pullups
Observations:
- behaviour may differ between IC units even within valid Vcc range,
- a given IC unit may change its behaviour within or near valid Vcc range,
- 175 class may pass at lower Vcc and fail at higher Vcc,
- S175 class may pass at higher Vcc and fail at lower Vcc.
Interesting the way 175 and S175 devices differ in their response to Vcc change.
I didn't see any of the fail at lower VCC on any S175 parts, but I only
tested one sample from each date code. I have more parts, but I got bored :-)
The group of TI S175s from 1979 did seem to show
reliable behaviour over the
entire range.
I found this to be true of the TI S175 date code 345B (which I don't know how
to decode) and the Signetics 175 from '82. Both were rock solid in the test
circuit and work reliably in the PDP-8m console socket with no add'l rework
(ie, no pullups required as with the previous S175 SIG part).
One way or the other, the DEC front panel is relying
on unspecified behaviour
of the device. I wonder if the designers were just relying on old habits of
setting flip-flops via collector triggering from the discrete days, and just
got lucky that it worked.
I agree, this circuit is more complex than it really needs to be. The two 175s
and the 04 hex inverter packs can be replaced by three 7400 quad nand packs (plus
six pullup resistors) that implement simple cross-coupled latch debouncers.
Sometimes a designer gets a bug in their head to implement a 'cute' circuit
when really doing something more simple and straightforward is the better solution.
Of course I would never do that :-) , I'm only reporting that I've seen it done.
Anyway, for now it appears replacing the existing flaky S175 dc '76 parts in my
PDP-8m console with SIG 175 dc '82 parts has fixed the problems, no add'l rework
or circuit hacks required.
Don
6845
days inactive
6847
days old
7 comments
5 participants
participants (5)
-
ajp166＠bellatlantic.net -
ak6dn＠mindspring.com -
healyzh＠aracnet.com -
hilpert＠cs.ubc.ca -
vrs＠msn.com